Some Android cellphone users noticed recently that Facebook has saved a virtual trove of their personal call data that extends back years.
Amid an outcry over the social network's handling of a controversy involving a third-party application's misuse of personal data, several Android users who pulled down archive data from Facebook found very detailed personal metadata. That information included call logs containing names, phone numbers, and the length of each call made.
In a statement given to Ars Technica, Facebook pointed out that the call log was "a widely used practice to begin by uploading your phone contacts."
The company clarified Sunday that it only collects that data on users who opt-in to the feature when installing Messenger or Facebook Lite.
"People have to expressly agree to use this feature. If, at any time, they no longer wish to use this feature they can turn it off in settings, or here for Facebook Lite users, and all previously shared call and text history shared via that app is deleted," the company said.
Facebook has come under mounting pressure from privacy advocates, advertisers and people who use the platform. On Sunday, the network was forced to take out ads in major publications to apologize for the data scandal.
Some users have even begun opting out of using the site altogether, although Facebook founder and CEO Mark Zuckerberg said last week that people weren't ditching the site in large numbers.
—CNBC's Sara Salinas contributed to this report.
Correction: This story was modified to correct the spelling of Ars Technica.